Abstract

Eukaryotic cell compartmentalization concentrates the required substrates and reagents, preventing deleterious side-reactions. The sub-organellar spatial-temporal organization within many organelles is known, but not the chloroplast. Photosynthetic biochemical reactions are well studied, conversely the organization of photosynthetic thylakoid membrane biogenesis is lacking. We studied the spatial-temporal patterning of thylakoid biogenesis under two distinct developmental model systems using Chlamydomonas reinhardtii; 1) chloroplast differentiation using the y-1 mutant and 2) mature chloroplast growth using diurnally-entrained cells. Immunoblot analysis uncovered the contrasting accumulation trends of photosynthetic electron transport chain complexes, correlating to their differing fluorescence microscopy in situ localizations. The chloroplastic T-zone region is now a privileged site for the synthesis and assembly of the chloroplast genome-encoded photosystem I and photosystem II subunits, as well as their chlorophyll pigment. Findings seen here also support the temporal regulation of the spatial T-zone region, which is related to thylakoid biogenesis within cells. A comparison of the two algal model systems has uncovered an increased capacity of chloroplast differentiation within mature algal cells. Finally, transmission electron microscopy results displayed candidate ultrastructures for the T-zone membrane platform. Therefore, using C. reinhardtii, we have expanded the spatial organization of thylakoid biogenesis, discovered temporal patterns, characterized potential T-zone related ultrastructures, and documented the T-zone within both differentiating and growing cultures.